It is often necessary to provide electric fuses with means which are responsive to blowing thereof. Such means may be used either to indicate that the particular fuse has blown, or to trip an automatic switch or circuit breaker arranged in series with the fuse.
As a general rule the means responsive to blowing of a fuse include a spring-biased pin normally maintained in a first, or non-indicating position, and caused when the fuse blows by its spring bias to move to a second, or indicating position.
Fuses of the kind under consideration include two parallel-connected fusible element means namely a first low resistance fusible element means and a second high resistance fusible element means. Normally the first fusible element means carries virtually the entire load current and the second fusible element means is virtually non-current carrying. When the current path through the first fusible element means is interrupted by overload currents, or short-circuit currents, the second fusible element means becomes current-carrying. This causes fusion thereof in rapid sequence to the overcurrent-caused fusion of the first fusible element means. Fusion of the second fusible element means liberates blown fuse responsive means allowing the same to move from their above referred-to first position to their above referred-to second position.
In low-voltage fuses the second or high-resistance fusible element establishes a current path which is normally continuous, extending from one terminal element of the fuse to the other terminal element thereof. It is common practice in high voltage fuses to include a break-down spark gap in the current path of the second or high-resistance fusible element. As a result of the presence of that break-down spark gap the current path of the second or high-resistance fusible element is normally interrupted at the point where the break-down spark gap is located. When the current path of the first or low resistance fusible element is interrupted on occurrence of an overload current or short-circuit current, a voltage appears across the break-down gap which voltage is sufficiently high to cause its break-down. As a result of this break-down and consequent arcing across the gap, a continuous current path is now established by the intermediary of the second or high resistance fusible element means from one terminal element of the fuse to its other terminal element. This causes fusion of the second fusible element at some predetermined point thereof which, in turn, causes operation of the blown fuse responsive device of the fuse.
It will be understood that in the aforementioned type of fuses the second or high resistance fusible element is subdivided into sections by the presence of the aforementioned spark gap. While the entire length of the high resistance shunt across the first low resistance fusible element may rightly be referred-to as a second fusible element, fusion thereof occurs, as mentioned above, only at a predetermined point of it at which point the positioning of the blown fuse indicating means is controlled by the mechanical integrity, or the absence of the mechanical integrity, respectively, of the second fusible element.
The present invention refers more specifically to fuses for elevated circuit voltages wherein the first low resistance fusible element is substantially helically wound around a mandrel of electric insulating materials, and one of the principal objects of this invention is to use that mandrel for purposes other than as fusible element support, thus greatly reducing the cost of manufacture of the fuse.
Another object of the invention is to adapt the aforementioned mandrel to define with a satisfactory degree of precision the break-down spark gap whose electric break-down causes energization of the high resistance fusible element, and operation of the blown fuse responsive means.
Still another object of this invention is to provide a high-voltage fuse wherein the reaction to the spring forces biasing the blown fuse responsive means are positively transmitted to the insulating mandrel. These spring forces may be very considerable, particularly if the blown fuse responsive means are not merely intended as blown fuse indicators, but are supposed to overcome the relatively large latch friction of a high voltage disconnect, or like piece of equipment. In the absence of firm and rugged means for anchoring the spring that biases the blown fuse responsive means, the latter may move simply by conventional handling of the fuse to a position intermediate their non-blown fuse position and their blown fuse position.
A further object of the invention is to provide fuses for elevated circuit voltages which have plug terminal inserted into the ends of the tubular fuse casing of insulating material wherein energization of the high resistance fusible element is controlled by the break-down of one or more spark gaps serially arranged therein, and wherein the blown fuse responsive means lends itself to be, and actually is, arranged in coaxial relation to the plug terminals.
Further objects and advantages of the invention will become apparent as the following description proceeds, and the features of novelty which characterize the invention will be pointed out with particularity in the claims annexed to, and forming part of, this specification.